A semiconductor wafer generally has a substrate surface on which one or more integrated circuits is formed. The substrate surface is desirably as flat, or planar, as possible before the surface is processed to form the integrated circuits. A variety of semiconductor processes such as for example photolithography are used to form the integrated circuits on the flat surface, during which the wafer takes on a defined topography. The topography is subsequently planarized, because an irregular surface, because the surface has an excess of material deposited thereon, or the surface havs imperfections which seriously impede subsequent fabrication processes. Thus, it is necessary to polish the wafer surface to render it as planar or uniform as possible and to remove surface imperfections.
CMP is now widely known to those skilled in the art and has been described in numerous patents and open literature publications. In a typical CMP process, a substrate (e.g., a wafer) is placed in contact with a rotating polishing pad attached to a platen. One method has the wafer held in place on a mount using negative pressure, such as vacuum, or hydrostatic or pneumatic pressure, where the mount is situated near or over a polishing pad. A CMP slurry, typically an abrasive and chemically reactive mixture, is supplied to the pad during CMP processing of the substrate. During the CMP process, the pad (fixed to the platen) and substrate are rotated while a wafer carrier system or polishing head applies pressure (downward force) against the substrate. The slurry accomplishes the planarization (polishing) process by chemically and mechanically interacting with the substrate film being planarized due to the effect of the rotational movement of the pad relative to the substrate. Polishing is continued in this manner until the desired film on the substrate is removed with the usual objective being to effectively planarize the substrate. For such a semiconductor wafer, a typical CMP process involves polishing the metal in a controlled manner to preferentially etch certain conductors, insulators or both over the oxide beneath the metal, such that the metal is substantially coplanar with the oxide and remains in the grooves or stud vias of the oxide. After CMP, the substantially coplanar surface is ready for further processing.
Economic forces are requiring the use of faster processing. One approach has involved increasing the downward pressure on the wafer carrier in order to increase material removal rates. This approach is generally disfavored as the requisite downward pressure is considered too high and too likely to cause wafer damage. Another approach has involved increasing the amount of oxidizing agent used in the CMP slurry in an effort to increase chemical removal of targeted material. This approach is largely disfavored as the use of increased amounts of oxidizing agents increase material costs and also detrimentally add to the handling issues and environmental issues associated with many oxidizing agents and also increase costs. Additional approaches have involved using various protected combinations of oxidizers, chelators, corrosion inhibitors, solvents, and other chemicals in the slurry, various abrasives including for example a zirconium abrasive or mixed abrasives, and/or using point-of-use mixing techniques. These approaches are generally undesirable, as they typically complicate CMP in terms of tooling and process control for example, consume more process time, and/or increase costs.
It is generally known that oxidizers admixed in a solution can provide synergistic etching rates. While ferric salts, cerium salts, peroxides, persulfates, or hydroxylamines form the oxidizing capacity of most commercially available CMP slurries, those of ordinary skill in the art have long known that certain of these oxidizers can be admixed with others in this group and also with other oxidizers, and the resulting composition can show synergistic results. For example, the compositions claimed in U.S. Pat. No. 6,117,783 to Small et al., which claims priority to a provisional application filed Jul. 25, 1996, the contents of which is incorporated herein by reference thereto, claims a CMP slurry having a hydroxylamine compound and hydrogen peroxide, and teaches in the specification that the two have a synergistic effect.
Many slurries use a metal ion, typically Fe ions or Ce ions, as an oxidizer, alone or in combination with another oxidizer. However, both iron and cerium, as well as other metal ions, causes metallic contamination of substrates. U.S. Pat. No. 5,773,364 describes a CMP slurry where oxidizers include ferric nitrate or cerium nitrate, and note the problem that metal ions are created as a result of the oxidizing process. U.S. Pat. No. 5,958,288 suggests limiting the amount of ferric nitrate to from about 0.001 to about 2.0 weight percent, where the slurry comprises another oxidizer The ferric ion contamination may be exceedingly difficult to subsequently remove. Raghunath et al showed in Mechanistic Aspects Of Chemical Mechanical Polishing Of Tungsten Using Ferric Ion Based Alumina Slurries, in the Proceedings of the First International Symposium on Chemical Mechanical Planarization, 1997, that alumina slurries containing ferric salts is conducive to the formation of an insoluble layer of ferrous tungstate on tungsten. The industry has developed methods to remove a portion of the metallic contamination, for example by: physical desorption by solvents; changing the surface charge with either acids or bases so that Si—OH or M—OH group can be protonated (made positive) in acid or made negative with bases by removing the proton; ion competition, for example removing adsorbed metal ions by adding acid (i.e. ion exchange); subsequent oxidation of metals to change the chemical bonds between the impurities and substrate surface; and subsequent etching the surface, wherein the impurity and a certain thickness of the substrate surface is removed, as described in U.S. Pat. No. 6,313,039. There have been various post-CMP cleaners developed to remove metallic contamination, but removal of all undesired metal ions is substantially beyond the range of cleaners, and as the size of the structures continues to decrease, even a very small number of metallic atoms deposited on a surface will result in undesired shorts or current leakage. Additionally, metal ion-containing fluids and many post-CMP cleaners are environmentally undesirable and expensive treatment may be needed prior to waste disposal of used product.
Another problem with many soluble metal oxidizers is that they react with and cause degradation of other oxidizers. When a metal-containing oxidizer is admixed with a non-metal-containing oxidizer, for example hydrogen peroxide in a solution, the two often react in an undesirable fashion, and the oxidizing capacity of the mixture declines rapidly, but without any rigorous predictability, with time. For example, ferric nitrate reacts with hydrogen peroxide in CMP formulations at essentially all usable pHs, making the formulation oxidizing capacity fall with time, which complicates polishing since there is a non-uniformity problem, and also causing formation of undesired products. It is known that if the pH is above about 5, iron precipitates as Fe(OH)3 which rapidly catalytically decomposes hydrogen peroxide to oxygen, without forming hydroxyl radicals.
Therefore, despite the known advantages of having multiple oxidizers, for example a metal-containing oxidizer admixed with either another metal-containing oxidizer or with a non-metal-containing oxidizer, there has been a tendency in the industry to reduce the amount of metal ions in CMP slurries. For example, Rodel, a large commercial manufacturer of CMP slurries that at point of use are designed to be used with non-metal-containing oxidizers such as peroxides and persulfates, had about 30 ppm of metals, primarily iron, in the liquid portion of an MSW1000™ slurry produced in 1995. While this iron would have functioned as a promoter, it is likely the manufacturer did not add the iron, but rather the iron was in the solution as a result of impurities. Later generations of Rodel slurries, for example the Rodel MSW1500™ slurry that was sold in 2002, has only 4.2 ppm iron, according to their web-site. Even where soluble iron ions, e.g., ferric nitrate, are added to increase rates, such as described in U.S. Pat. No. 5,958,288, the preferred amount of ferric nitrate added to a hydrogen peroxide solution is very small, that is, 0.01 to about 0.05 weight percent, or about 100 ppm to about 500 ppm. Another method of reducing metallic contamination is to use sequential CMP polishing steps using sequential formulations that have decreasing amounts of metal, so that metal deposited from earlier formulations in a CMP process are removed by CMP with subsequent formulations that are metal-free. For example, a Rodel CMP slurry, the MSW2000™, has a first formulation having 12 ppm Fe, and a second formulation that has less than 0.3 ppm Fe. However, use of sequential formulations adds additional costs to processing, as well as adding complexity to the required equipment.
There is another mechanism for synergy that has been described in co-owned U.S. published applications 20040029495, 20040006924, and 20030162398, the disclosures of which are incorporated herein by disclosure thereto. In these applications, various metals are absorbed onto abrasives in an ionic form. The synergy is based on Fenton's reaction, where the relatively benign oxidizers generate very strong, short-lived, non-organic, oxygen-containing free radicals. The classic Fenton's reaction is the production of free radicals as a byproduct of the oxidation of soluble ferrous ions by hydrogen peroxide. The useful pH for classical Fenton's reaction utilizing soluble iron ions is pH 3 and pH 6, particularly 4 to 5.
It is clear that the industry is moving away from metals, for example iron, in the fluids. Also, when iron or other metal-containing formulation is admixed with non-metal-containing oxidizers, the “pot-life” of the formulation is very short, so mixing is generally point-of-use mixing, which complicates CMP processes and equipment and can create start-up problems even after a temporary interruption on the processing. Further developments in the field of CMP technology are desired.